The present invention relates to a process for producing fiber-reinforced resin moldings and, more particularly, to a process for producing fiber-reinforced resin moldings having few cells by structural reaction injection molding (SRIM).
SRIM is a technique for producing fiber-reinforced resin moldings according to the reaction injection molding (RIM) method.
An ordinary practical mode of SRIM is illustrated in FIG. 4. According to this SRIM method, fiber-reinforced resin moldings are produced in the following way.
A fiber reinforcement 11 is placed in the cavity 8 of a mold 3 consisting of top force 1 and bottom force 2. After closing the mold, a liquid resin material 12 is injected into the cavity and cured. After curing has been completed, the mold 3 is opened and the produced molding is withdrawn therefrom.
Injection of the liquid resin material is performed by impingement-mixing the starting liquid resin materials by a mixing head 4 and then passing the mixed material into the mold cavity 8 through a resin material discharged port 5, a runner 6 and a gate 7 under an injection pressure of usually about 0.5 kg/cm.sup.2 G. In the inside of the mixing head 4 are provided the circulation paths (not shown) prepared for the respective components of the resin material, and in these circulation paths are disposed the orifices opposing to each other. Mixing of the liquid resin materials is accomplished by the instantaneous impingement-mixing of the resin materials when passing through the orifices.
In the moldings obtained by RIM, it is often practiced to blend a foaming agent or other additives or to blow a gas into the resin materials for attaining a reduction of weight of the moldings or for preventing dimensional shrinkage on curing, thereby making many tiny cells contain in the moldings. Such method is carried out in production of RIM moldings of polyurethane, polyurea, polyamide and the like.
In the case of the SRIM moldings, on the other hand, it is usually undesirable to contain the cells, because these moldings, unlike the RIM moldings, are used mainly for structural materials for which strength and stiffness are important factors. Further, the presence of the cells lowers impregnation of the liquid resin material into the fiber reinforcement and also impairs transparency of the moldings to reduce their commercial value.
However, even if SRIM molding is carried out without blending any foaming agent, the resulting moldings tend to contain the cells, due to the presence of cells in the liquid resin material, vapors of the said resin material and air in the mold, in case of using the ordinary method.
The object of the present invention is to provide a process for producing fiber-reinforced resin moldings having few cells or voids by SRIM.
As a result of extensive studies for attaining the said object, it has been found that by bringing the interior of the mold cavity into which a liquid resin material has been injected, into a pressurized state by introduction of a pressurized gas, in the process for production of fiber-reinforced resin moldings by SRIM, cells in the moldings has been remarkably reduced in number. The present invention has been attained on the basis of this finding.